Power devices are widely used to carry large currents and support high voltages. Modern power devices are often fabricated from monocrystalline silicon semiconductor material. One widely used power device is the power Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In a power MOSFET, a control signal is supplied to a gate electrode that is separated from the semiconductor surface by an intervening insulator, which may be, but is not limited to, silicon dioxide. Current conduction occurs via transport of majority carriers, without the presence of minority carrier injection that is used in bipolar transistor operation. Power MOSFETs can provide an excellent safe operating area, and can be paralleled in a unit cell structure.
As is well known to those having skill in the art, power MOSFETs may include a lateral structure or a vertical structure. In a lateral structure, the drain, gate and source terminals are on the same surface of a substrate. In contrast, in a vertical structure, the source and drain are on opposite surfaces of the substrate.
One widely used silicon power MOSFET is the double diffused MOSFET (DMOSFET) which is fabricated using a double-diffusion process. In these devices, a p-base region and an n+ source region are diffused through a common opening in a mask. The p-base region is driven in deeper than the n+source. The difference in the lateral diffusion between the p-base and n+source regions forms a surface channel region. An overview of power MOSFETs including DMOSFETs may be found in the textbook entitled “Power Semiconductor Devices” by B. J. Baliga, published by PWS Publishing Company, 1996, and specifically in Chapter 7, entitled “Power MOSFET”, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
Another type of widely used power device is the Bipolar Junction Transistor (BJT). A BJT typically includes a semiconductor material having two opposing p-n junctions in close proximity to one another; thus, BJTs may be referred to as “n-p-n” or “p-n-p” transistors. In operation, current carriers enter a region of the semiconductor material of a first conductivity type adjacent one of the p-n junctions, which is called the emitter. Most of the charge carriers exit the device from a region of the semiconductor material of the first conductivity type adjacent the other p-n junction, which is called the collector. A small portion of semiconductor material known as the base, having a second conductivity type, opposite the first conductivity type of the collector and the emitter, is positioned between the collector and the emitter. The two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter.
When current is injected into or extracted from the base, depending upon whether the BJT is n-p-n or p-n-p, the flow of charge carriers, i. e., electrons or holes, which can move from the emitter to the collector, may be affected. Typically, small currents applied to the base can control proportionally larger currents passing through the BJT, giving it usefulness as a component of electronic circuits. Structural and operational details of BJT's are discussed in Solid State Electronic Devices by B. Streetman (2nd edition (1980), chapter 7), the content of which is incorporated herein by reference as if set forth in its entirety.
Insulated Gate Bipolar Transistors (IGBTs) are yet another type of power device that can combine the drive gate characteristics of the power MOSFET with the high current and low saturation voltage of BJTs by combining a power MOSFET for the control input and a bipolar power transistor as a switch, in a single device. An overview of IGBTs may be found in the above-cited textbook entitled “Power Semiconductor Devices” and specifically in Chapter 8, entitled “Insulated Gate Bipolar Transistor”, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
Recent development efforts in power devices have also included investigation of the use of silicon carbide (SiC) devices for power devices. Silicon carbide has a wide bandgap, a lower dielectric constant, a high breakdown field strength, a high thermal conductivity, and a high saturation electron drift velocity compared to silicon. These characteristics may allow silicon carbide power devices to operate at higher temperatures, higher power levels and/or with lower specific on-resistance than conventional silicon-based power devices. A theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar et al. entitled “Comparison of 6H-SiC, 3C-SiC and Si for Power Devices”, IEEE Transactions on Electron Devices, Vol. 40, 1993, pp. 645 655. A power MOSFET fabricated in silicon carbide is described in U.S. Pat. No. 5,506,421 to Palmour entitled “Power MOSFET in Silicon Carbide” and assigned to the assignee of the present invention. Other power devices fabricated in silicon carbide are described in U.S. Pat. Nos. 7,118,970; 7,074,643; 7,026,650; 6,979,863 and 6,956,238.
Silicon carbide IGBTs may be highly desirable when very high blocking voltages are desirable. In particular, because the on-resistance of a unipolar power device such as a DMOSFET generally increases by the square of the blocking voltage, it may be desirable to provide IGBT devices at very high blocking voltages. For silicon carbide devices, this transition point may occur at about 10 kV when considering both conduction and switching losses. As is well known to those having skill in the art, a unipolar device such as a DMOSFET may be converted to a bipolar device, such as an IGBT by adding a junction between the substrate and the epitaxial drift region. For example, a unipolar n-channel DMOSFET structure can become bipolar when the substrate is switched from n-type to p-type. The p-n junction so formed is forward biased in the on-state, and injects minority carriers into the lightly doped drift region, to increase its conductivity, a phenomenon known as “conductivity modulation”. Thus, a p-channel IGBT may be fabricated on n-type substrates of, for example, 4H silicon carbide, to provide a p-n junction between the n -type substrate and the p-type drift region.
It may also be desirable to provide an n-channel IGBT, because this device could provide lower on-resistance and/or higher blocking voltage than its p-channel counterpart. Moreover, n-channel devices, with their positive voltage polarities and similarities to conventional power MOSFETs, may be more attractive from a systems point of view. Unfortunately, 4H silicon carbide p-type substrates that would be used to fabricate n-channel IGBTs generally lack both the quality and conductivity to work well in an IGBT.